Conventional film (or paper) processing devices do not provide a sufficient level of agitation at the film liquid interface. As a result, a layer of liquid that is depleted of reactants and enriched in reaction by products exists at the film surface. This layer is the chemical boundary layer. During photographic processing, this boundary layer can influence both the rate at which photochemicals are transferred to and from the film, as well as their concentrations within the film. Either influence can affect the rate and quality of processing.
Analysis and experimental measurements of conventional processors indicate that boundary layers exist which are thick enough so as to become the processing rate limiting parameter. More specifically, the transfer of chemical mass and heat energy through the chemical and thermal boundary layers occurs more slowly than transfer through the film itself. This condition results in low processing speeds, excessively long processing paths and increased size of the processor, including the dryer section. Also, the chemical concentrations in the processor boundary layer need to be excessively high to maintain reasonable mass transport rates resulting in inefficient utilization of the processing chemicals.